The present invention relates to a power supply unit comprising a multiplicity of cells such as lithium secondary cells, nickel hydrogen cells, lead seal cells, electric double layer capacitors and fuel cells connected in series parallel, and a distributed power supply system and an electric vehicle including them.
In the case where a plurality of cells are connected in series, the variations of capacitance, initial voltage and temperature from one cell. to another causes a different voltage for a different cell, thereby making it difficult for all the series-connected cells to share the voltage across the circuit uniformly.
Especially in the case where the lithium secondary cells or the electric double layer capacitors employing an organic solvent as an electrolytic solution are connected in series, voltage variations causes an overcharge or an overdischarge, often resulting in a rupture or a fire, or at least an overcharge or an overdischarge, which poses the problem of an extremely shortened service life of the cells.
In order to prevent the overcharge or overdischarge, the charge/discharge operation may be performed with a pre-set protective level. In charge mode, however, the charge operation stops when the voltage across a high-voltage cell has reached the protective level. As a result, the remaining low-voltage cells fail to be fully charged before the end of the charge operation.
In similar fashion, the discharge operation stops at the time point when the voltage across a low-voltage cell has reached a protective level. As a result, the remaining high-voltage cells cannot be fully discharged before the end of the discharge operation.
In the series connection of cells, therefore, the charge/discharge time becomes shorter than in the case where each cell is charged/discharged independently.
In a conventional battery charging apparatus intended to solve this problem, the charge current supplied through a bypass is changed by a current changing means progressively according as the voltage across the cells being charged approaches a set value thereby to set the cells into a uniform state. Examples are illustrated in U.S. Patent No. 5,557,189 and a corresponding Japanese Patent No. JP-A-7-230829. FIG. 12 is a diagram showing such a battery charging apparatus. In FIG. 12, reference numerals 1101a to 1101c designate cells, numerals 1102a to 1102c voltage detection means, numeral 1103 set voltage application means, numerals 1104a to 1104c comparison control means, and numerals 1105a to 1105c current changing means. The circuit for the cell 1101a is so configured that the voltage detection means 1102a, the comparison control means 1104a and the current changing means 1105a are connected in parallel to each other, and the set voltage application means 1103 applies a set voltage indicating the setting of a voltage value of the cell 1101a. 
The present voltage value of the cell 1101a is detected by the voltage detection means 1102a, and compared in the comparison control means 1104a with the set value of the voltage applied by the set voltage application means 1103a. According as the present cell voltage approaches the set voltage value, the charge current flowing in the current changing means is increased progressively. Specifically, the charge current to the cell 1101a is controlled progressively downward. In this way, an overcharge is prevented.
The fact about the cell 1101a described above equally applies to the cell 1101b and the cell 1101c. In other words, the voltage detection means 1102b, the comparison control means 1104b and the current changing means 1105b for the cell 1101b, and the voltage detection means 1102c, the comparison control means 1104c and the current changing means 1105c for the cell 1101c, work exactly the same manner as the corresponding means, respectively, of the cell 1101a. 
Another example of the prior art is disclosed in JP-A-2000-78768. This is intended to correct the variations caused at the time of charging the lithium ion secondary cell and to prevent the trouble such as overcharge for an improved service life. Specifically, a negative electrolytic solution circulation pump and a positive electrolytic solution circulation pump are used for correcting the variations of the charge/discharge operation. Still another example of the prior art is disclosed in JP-A-2000-511398. This is a system for equalizing the cells and is a combination of energy storage elements that can be switched. Specifically, the charge is shifted between batteries each including a plurality of cells connected in series. The charge is pulled out of a particular battery of a higher voltage and transferred to another battery of a lower voltage.
In the conventional battery charging apparatus, a cell voltage at the time of charging is compared with a set value, and with the approach of the cell voltage to the set voltage value, the charge current is progressively diverted to the current changing means in parallel to the cells thereby to assure uniform conditions of the cells.
According to the prior art, however, the amount of current that can be diverted is greatly limited by the heat generated in the current changing means. Thus, the effect of obviating the voltage variations among the cells is reduced. The current changing means having a large thermal capacitance through which a large current can flow, on the other hand, is large in size and the system becomes bulky. Also, an electrical circuit other than the cells is required and increases the cost. The method of circulating the electrolytic solution, on the other hand, requires a pump. Also, a battery equalizer including a switch circuit for moving the charge by switching and a control circuit for the switch circuit is required.